Numerous image/video editing techniques are currently available so that realistic synthetic images/videos can be produced that appear to be realistic to a person. With skillful human interaction, many synthesized images/videos are difficult in being distinguished from real ones even by close visual examination. While greatly enriching user experience and reducing production cost, synthetic images/videos that appear to be realistic may also have adverse consequences. In assessing the validity of an image, one must determine if the image reflects a real situation and correct information. A photograph may be doctored to be misleading. One example is the “B. Walski event.” During the Iraq war, various newspapers took liberties that conflicted with reality. On Apr. 1, 2003, for example, the front page of the Los Angeles Times featured an image by photographer Brian Walski of a British soldier and Iraqi civilians outside Basra. (The Editorial Eye, Vol. 26, No. 8, August 2003.) The photo also appeared prominently in the Chicago Tribune, the Hartford Courant, and many other papers. The photo was identified as a fake by an alert Hartford Courant reader who noted a curious pattern of repetition in the crowd.
At first glance, watermarking may appear to be a solution for preventing doctored images. However, it is not a complete solution. First, doctored image/video detection is different from digital rights management, which is the intended objective of watermarking. The former aims at determining whether an image/video is real or not, where every image component may belong to the same owner. However, the latter aims at determining whether an image/video belongs to an owner, even though the image/video may be synthesized. Second, as commodity digital/video cameras do not supply the functionality of injecting watermarks as soon as the images/videos are captured, people may find it inconvenient to protect their photos by injecting watermarks on computers. Consequently, there are a very large number of images/videos without watermarks. Third, it is uncertain whether watermark can sustain heavy editing that is beyond simple copy/paste.
Another approach in the prior art is to test the statistics of an images. For example, the approach may test the interpolation relationship among nearby pixels. This approach may include checking the Color Filter Array (CFA) interpolation relationship among the nearby pixels. The approach may be effective in some aspects but is by no means always reliable or provide a complete solution. For example, the resampling test fails when the two images are not resampled or are resampled with the same scale. The double quantization effect does not happen if component images are similarly compressed. Blind gamma estimation and the signal to noise (SNR) test may fail when the component images come from the same camera or the kurtoses of the noiseless image and noise are not known a priori. And the CFA checking may require the a priori knowledge of de-mosaicing algorithm of the camera.
Although the blind gamma estimation method may also recover the gamma of the response function, typical camera response functions do not exactly follow a gamma curve. As a result, the estimated gamma may vary significantly on different parts of the image even when the image is original, making the detection unreliable. Moreover, the blind gamma estimation method tests regions of an image so that the Fourier transform can be applied. Also, in principle, the blind gamma estimation should compute the 4D bicoherence in order to detect the tampering on 2D images. Such computation is formidable. As a result, prior art may resort to row-wise (or column-wise) gamma estimation that only requires 2D bicoherence. Therefore, if the tampered region is surrounded by original regions, the tampering may not be detected. Unfortunately, this type of tampering is common.
With current image technologies, one can easily doctor an image by synthesis. To determine whether an image is doctored, methods and systems are therefore needed that provide efficient and reliable results.